[llvm] [AMDGPU][SDAG] Handle ISD::PTRADD in various special cases (PR #145330)

[Fabian Ritter via llvm-commits]

ritter-x2a wrote:

### Merge activity

* **Sep 19, 8:17 AM UTC**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/145330).


https://github.com/llvm/llvm-project/pull/145330